Connection structure for dispersion compensating optical fiber

ABSTRACT

In fusion-splicing a dispersion compensating optical fiber having a negative dispersion slope, with a connection optical fiber having a different near field pattern from that of the dispersion compensating optical fiber, if for the connection optical fiber, one is selected such that a theoretical joint loss in a used wavelength, obtained from an overlap integral of a near field pattern of the dispersion compensating optical fiber after fusion splicing and a near field pattern of the connection optical fiber after fusion splicing is presumed to be 0.3 dB or less, in an unconnected state, a construction enabling connection at a low loss results.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a connection structure between asingle-mode optical fiber and a dispersion compensating optical fiber,and more specifically, relates to a connection structure between asingle-mode optical fiber and a dispersion compensating optical fiberhaving a negative dispersion slope.

[0003] 2. Description of Related Art

[0004] A transmission line which combines a single mode optical fibersuch as one for use at 1.3 μm and a dispersion compensating opticalfiber for compensating the chromatic dispersion thereof, has heretoforebeen put to practical use.

[0005] The single mode optical fiber has a relatively large effectivecore area (Aeff), and hence nonlinear effects are suppressed. However,if a 1.55 μm band or the like is designated as the wavelength region foruse, chromatic dispersion increases. Therefore, a low loss transmissionline can be built up by compensating the chromatic dispersion with thedispersion compensating optical fiber.

[0006] Japanese Patent No. 2,951,562 discloses a structure forconnecting a normal single mode optical fiber and a dispersioncompensating optical fiber having a so-called W-type refractive indexdistribution shape for compensating the chromatic dispersion of thesingle mode optical fiber, via an intermediate optical fiber interposedtherebetween.

[0007] In this structure, the mode field diameter of the intermediateoptical fiber is made to have substantially the same value as the modefield diameter of the dispersion compensating optical fiber. Also, themode field diameter of this intermediate optical fiber on the singlemode optical fiber side is expanded so as to match the mode fielddiameter of the single mode optical fiber.

[0008] As a result, the joint loss between the intermediate opticalfiber, the dispersion compensating optical fiber and the single modeoptical fiber is reduced. Expansion of the mode field diameter isperformed by heating the end of the intermediate optical fiber todiffuse the dopant such as germanium added to the core thereof.

[0009] Recently, with the development of wavelength multiplextransmission, a dispersion compensating optical fiber which cancompensate not only the chromatic dispersion of the single mode opticalfiber but also the dispersion slope has been developed.

[0010] The dispersion slope is an inclination in the graph when thewavelength is plotted on the X-axis and the chromatic dispersion isplotted on the Y-axis, and the normal single mode optical fiber has apositive dispersion slope. Therefore, if the dispersion slope of thesingle mode optical fiber is compensated by the dispersion compensatingoptical fiber having a negative dispersion slope, a flat chromaticdispersion characteristic can be obtained in a relatively widewavelength region.

[0011] The normal single mode optical fiber referred to herein isassumed to be one normally used for propagating an optical signal, suchas one for use at 1.3 μm and a dispersion shifted optical fiber.

[0012] Moreover, the dispersion compensating optical fiber is preferablyone capable of single mode propagation.

[0013] However, the dispersion compensating optical fiber having such anegative slope has a different refractive index distribution shape fromthat of the dispersion compensating optical fiber of a type forcompensating only the chromatic dispersion which has been heretoforeproposed. Hence even if the connection structure is formed based on themode field diameter as disclosed in Japanese Patent No. 2,951,562, thejoint loss cannot be reduced. Particularly, when fusion splicing isperformed, there is a tendency for the increase in joint loss to becomeconspicuous.

[0014] As the dispersion compensating optical fiber, there has beenprovided one having an expanded effective core area, in order to preventdeterioration in the transmission quality due to the nonlinear effectwhich occurs therein. In this dispersion compensating optical fiberhaving an expanded effective core area, there is a tendency for theincrease in joint loss due to fusion splicing to further increase.

BRIEF SUMMARY OF THE INVENTION

[0015] In view of the above problems, it is an object of the presentinvention to provide a structure which can connect at a low loss adispersion compensating optical fiber of a dispersion slope compensationtype having a negative dispersion slope, with an other optical fibersuch as a connection optical fiber.

[0016] It is an other object of the present invention to provide astructure which can perform connection at a low loss, in the case wherea dispersion compensating optical fiber having a negative dispersionslope is connected to one end of this connection optical fiber, and asingle mode optical fiber whose dispersion slope is compensated by thedispersion compensating optical fiber is connected to the other endthereof.

[0017] As a result of intensive study by the present inventors to solvethe above described problems, it has been found that by using an opticalfiber of which the near field pattern after fusion splicing matches withthe near field pattern after fusion splicing of the dispersioncompensating optical fiber having a negative dispersion slope, the jointloss between this optical fiber and the dispersion compensating opticalfiber can be reliably reduced.

[0018] That is to say, a first aspect of the present invention is aconnection structure for a dispersion compensating optical fiberobtained by fusion-splicing a dispersion compensating optical fiberhaving a negative dispersion slope with a connection optical fiberhaving a different near field pattern from that of the dispersioncompensating optical fiber, wherein the connection optical fiber has anear field pattern such that a theoretical joint loss in a usedwavelength obtained from an overlap integral of a near field pattern ofthe dispersion compensating optical fiber after fusion splicing and anear field pattern of the connection optical fiber after fusion splicingis 0.3 dB or less, in an unconnected state.

[0019] A second aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiber, aneffective core area of the connection optical fiber in the unconnectedstate is larger than that of the dispersion compensating optical fiberin the unconnected state.

[0020] A third aspect of the present invention is that in the connectionstructure for a dispersion compensating optical fiber, an expansion rateof an effective core area due to heating of the connection optical fiberis smaller than that of an effective core area due to heating of thedispersion compensating optical fiber.

[0021] A fourth aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiber, thedispersion compensating optical fiber is connected to one end of theconnection optical fiber, and an other end of the connection opticalfiber is connected to a single mode optical fiber having a positivedispersion slope which is compensated by the dispersion compensatingoptical fiber.

[0022] A fifth aspect of the present invention is that in the connectionstructure for a dispersion compensating optical fiber according to thefourth aspect, when an effective core area of the dispersioncompensating optical fiber in the unconnected state is designated as A,an effective core area of the connection optical fiber in theunconnected state is designated as B, and an effective core area of thesingle mode optical fiber in the unconnected state is designated as C, arelation thereof is A<B<C.

[0023] A sixth aspect of the present invention is that in the connectionstructure for a dispersion compensating optical fiber according toeither one of the fourth and fifth aspects, when an expansion rate of aneffective core area due to heating of the dispersion compensatingoptical fiber is designated as D, an expansion rate of an effective corearea due to heating of the connection optical fiber is designated as E,and an expansion rate of an effective core area due to heating of thesingle mode optical fiber is designated as F, a relation thereof isF<E<D.

[0024] A seventh aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiberaccording to any one of the fourth to the sixth aspects, an effectivecore area in a used wavelength of the single mode optical fiber is from100 to 150 μm².

[0025] An eighth aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiberaccording to any one of the fourth to the sixth aspects, an effectivecore area in a used wavelength of the single mode optical fiber is from55 to 90 μm².

[0026] A ninth aspect of the present invention is that in the connectionstructure for a dispersion compensating optical fiber, the connectionoptical fiber comprises a core and a cladding provided on acircumference of the core, and fluorine is added to the cladding.

[0027] A tenth aspect of the present invention is that in the connectionstructure for a dispersion compensating optical fiber according to theninth aspect, the addition of the fluorine is at least 0.6% by weight.

[0028] An eleventh aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiberaccording to either one of the ninth and tenth aspects, the claddingcomprises at least two layers, and an outermost layer of the claddingcomprises pure silica.

[0029] A twelfth aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiber, thedispersion compensating optical fiber comprises a center core, a sidecore provided on a circumference of the center core, and a claddingprovided on a circumference of the side core, and a refractive index ofthe center core is higher than that of the cladding, and a refractiveindex of the side core is lower than that of the cladding.

[0030] A thirteenth aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiberaccording to the twelfth aspect, an effective core area in a usedwavelength of the dispersion compensating optical fiber is 16 μm² orlarger.

[0031] A fourteenth aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiberaccording to any one of the first to the eleventh aspects, thedispersion compensating optical fiber comprises a center core, a sidecore provided on a circumference of the center core, a ring coreprovided on a circumference of the side core, and a cladding provided ona circumference of the ring core, and a refractive index of the centercore and the ring core is respectively higher than that of the cladding,and a refractive index of the side core is lower than that of thecladding.

[0032] A fifteenth aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiberaccording to the fourteenth aspect, an effective core area in a usedwavelength of the dispersion compensating optical fiber is 18 μm² orlarger.

[0033] A sixteenth aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiberaccording to the fifteenth aspect, the dispersion compensating opticalfiber has characteristics shown in the following (a-1) to (d-1):

[0034] (a-1) a chromatic dispersion value in the used wavelength is from−60 to −45 ps/nm/km;

[0035] (b-1) a dispersion slope in the used wavelength is from −0.180 to−0.135 ps/nm/km;

[0036] (c-1) the effective core area in the used wavelength is from 20to 35 μm²; and

[0037] (d-1) a transmission loss in the used wavelength is 0.35 dB/km orless.

[0038] A seventeenth aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiberaccording to the fifteenth aspect of the present invention, thedispersion compensating optical fiber has characteristics shown in thefollowing (a-2) to (d-2):

[0039] (a-2) a chromatic dispersion value in the used wavelength is from−100 to −80 ps/nm/km;

[0040] (b-2) a dispersion slope in the used wavelength is from −0.300 to−0.230 ps/nm²/km;

[0041] (c-2) the effective core area in the used wavelength is from 18to 24 μm²; and

[0042] (d-2) a transmission loss in the used wavelength is 0.40 dB/km orless.

[0043] An eighteenth aspect of the present invention is that in theconnection structure for a dispersion compensating optical fiberaccording to the fifteenth aspect of the present invention, thedispersion compensating optical fiber has characteristics shown in thefollowing (a-3) to (d-3):

[0044] (a-3) a chromatic dispersion value in the used wavelength is from−45 to −35 ps/nm/km;

[0045] (b-3) a dispersion slope in the used wavelength is from −0.150 to−0.100 ps/nm²/km;

[0046] (c-3) the effective core area in the used wavelength is from 26to 35 μm²; and

[0047] (d-3) a transmission loss in the used wavelength is 0.25 dB/km orless.

[0048] A nineteenth aspect of the present invention is a transmissionline having the above described connection structure for a dispersioncompensating optical fiber.

[0049] A twentieth aspect of the present invention is a dispersioncompensator having the above described connection structure for adispersion compensating optical fiber.

[0050] A twenty-first aspect of the present invention is a connectionmethod for a dispersion compensating optical fiber for obtaining theabove described connection structure for a dispersion compensatingoptical fiber.

[0051] According to the above aspects, by using the connection opticalfiber whose near field pattern after fusion splicing is matched, thedispersion compensating optical fiber having a negative dispersion slopecan be connected with the connection optical fiber at low loss.

[0052] As a result, the dispersion compensating optical fiber can beconnected with a single mode optical fiber which is compensated by thisdispersion compensating optical fiber via the connection optical fiber,at low loss.

[0053] Also in the cladding of the connection optical fiber, the jointloss with a single mode optical fiber having a particularly largeeffective core area can be reduced, by forming at least a layer adjacentto the core from a fluorine-added silica glass.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0054]FIG. 1 is a graph showing changes in the near field pattern of adispersion compensating optical fiber due to heating;

[0055]FIG. 2 is a graph showing a W-type refractive index distributionshape, as one example of a single mode optical fiber suitable for theconnection structure of the present invention;

[0056]FIG. 3 is a graph showing a W-type refractive index distributionshape with a segment core, as one example of a dispersion compensatingoptical fiber suitable for the connection structure of the presentinvention; and

[0057]FIG. 4 is a graph showing near field patterns of various opticalfibers.

DETAILED DESCRIPTION OF THE INVENTION

[0058] One example of the present invention will be described below.

[0059] In the present invention, for the used wavelength, a suitablewavelength region is preferably selected from 1.53 to 1.63 μm, from thestandpoint of transmission characteristics. In the case of performingwavelength multiplex transmission, a relatively wide wavelength regionis selected.

[0060]FIG. 1 is a graph showing changes in the near field pattern at thetime of fusion splicing of a dispersion compensating optical fiberhaving a negative dispersion slope. The near field pattern can bemeasured by a method described in G.650 specified in the ITUT Standard.Specifically, for example, the near field pattern can be obtained bymeasuring a far field pattern and subjecting this measurement result toan inverse Fourier transform. The near field pattern is expressed by anoptical power distribution.

[0061] In this graph, the X-axis plots a radius of the optical fiber,and the Y-axis plots the optical power observed at the time ofmeasurement. The scale in the Y-axis is dBm, which is normalized by theoptical power.

[0062] In the graph, 0, 1000 ms (milli-seconds) and 1800 ms show heatingtime at the time of fusion splicing. Conditions such as the temperature,other than at the time of heating, are constant. Here 0 shows a nearfield pattern before fusion splicing.

[0063] As shown in this graph, in the dispersion compensating opticalfiber having a negative dispersion slope, the near field pattern iseasily changed due to heating at the time of fusion splicing.

[0064] In the normal optical fiber, such a change in the near fieldpattern occurs. However, the change in the dispersion compensatingoptical fiber having the negative dispersion slope is conspicuous.

[0065]FIG. 2 and FIG. 3 are explanatory diagrams respectively showingexamples of refractive index distribution shapes of dispersioncompensating optical fibers having negative dispersion slopes.

[0066]FIG. 2 shows a so-called W-type refractive index distributionshape, and in this refractive index distribution shape, a core 13 isformed of a center core 11 provided in a center and a side core 12provided in a concentric circular shape on the circumference thereof,and a cladding 15 is provided in a concentric circular shape on thecircumference of the core.

[0067] The relation between refractive indexes of these is such that therefractive index of the center core 11 is higher than that of thecladding 15, and the refractive index of the side core 12 is lower thanthat of the cladding 15.

[0068] The refractive index is adjusted by the addition of a dopant suchas germanium having an action of raising the refractive index, orfluorine having an action of lowering the refractive index.

[0069] For example, the center core 11 comprises a germanium-addedsilica glass, the side core 12 comprises a fluorine-added silica glass,and the cladding 15 comprises a pure silica glass or a fluorine-addedsilica glass.

[0070] The dispersion compensating optical fiber having a negativechromatic dispersion and a negative dispersion slope can be obtained byadjusting the relative index difference Δ₁ of the center core 11 basedon the cladding 15, the relative index difference Δ₂ of the side core 12based on the cladding 15, and the ratio between the radius r₁ of thecenter core 11 and the radius r₂ of the side core 12.

[0071]FIG. 3 shows a so-called W-type refractive index distributionshape with a segment, and this refractive index distribution shape isformed by a core 23 comprising a central center core 21, a side core 22,and a ring core 24 provided in a concentric circular shape in order, anda cladding 25 provided in a concentric circular shape on thecircumference thereof.

[0072] The refractive index of the center core and the ring core 24 isrespectively set higher than that of the cladding 25, and the refractiveindex of the side core 22 is set lower than that of the cladding 25.

[0073] In this example, the refractive index of the ring core 24 islower than that of the center core 21.

[0074] For example, the center core 21 and the ring core 24 comprise agermanium-added silica glass, the side core 22 comprises afluorine-added silica glass, and the cladding 25 comprises a pure silicaglass or a fluorine-added silica glass.

[0075] The dispersion compensating optical fiber having a negativechromatic dispersion and a negative dispersion slope can be obtained byadjusting the relative index difference Δ₁₁ of the center core 21 basedon the cladding 25, the relative index difference Δ₁₂ of the side core22 based on the cladding 25, the relative index difference Δ₁₃ of thering core 24 based on the cladding 25, the ratio between the radius r₁₁of the center core 21 and the radius r₁₂ of the side core 22 and theratio between the radius r₁₁ of the center core 21 and the radius r₁₃ ofthe ring core 24.

[0076] As a result of the study by the present inventors, in thedispersion compensating optical fiber having a negative dispersionslope, the reason why the change in the near field pattern due toheating is conspicuous is as described below.

[0077] A dopant having an action of raising the refractive index, suchas germanium, is generally added not only to the dispersion compensatingoptical fiber having a negative dispersion slope but also to the core ofthe optical fiber. Since germanium is generally used preferably, thedescription below will be made taking germanium as an example.

[0078] Also, when an optical fiber is drawn from the base material ofthe fiber, and cooled and solidified, a stress resulting from adifference in the glass transition point of the constituent materialsfor each layer which constitutes the optical fiber, is frozen in theoptical fiber (mainly in the core). This is referred to as residualstress.

[0079] Then, if the optical fiber is heated, germanium added to the corediffuses towards the cladding. The residual stress is also released dueto softening of the constituent material by heating.

[0080] As described above, since germanium has an action of raising therefractive index, the effective refractive index of the core decreasesdue to the diffusion of germanium. The effective refractive index of thecore also decreases due to release of the residual stress. As a result,confinement of light in the core becomes weak, thereby expanding theeffective core area and also expanding the near field pattern.

[0081] In the dispersion compensating optical fiber having a negativedispersion slope, the relative index differences Δ₁, Δ₁₁ of the centercores 11 and 21 shown in FIGS. 2 and 3 are respectively for example 1.0%or higher, which is relatively high. Therefore, at the time of drawingthe optical fiber from the base material of the fiber, it is necessaryto suppress an increase in the transmission loss by increasing thedrawing tension, and this tension becomes the residual stress.Accordingly, the residual stress increases.

[0082] On the circumference of the center cores 11 and 21, there aregenerally provided side cores 12, 22 added with fluorine. If fluorine isadded in a layer adjacent to a layer comprising a germanium-added silicaglass, diffusion of germanium is promoted.

[0083] Since the large residual stress is released by heating at thetime of fusion splicing, and germanium in a relatively large amount isreadily diffused, a decrease in the effective refractive index becomeslarge.

[0084] When the effective refractive index of the center cores 11, 21decreases, confinement of the light in the cores 13, 23 is weakened,thereby expanding the effective core area and the near field patternoutwards of the cores 13, 23, respectively. When heating is continued,and expansion of the near field pattern progresses, and coupling betweenthe propagation mode propagating the cores 13, 23 and the cladding modeoccurs, thereby further increasing the joint loss.

[0085] Therefore, in the dispersion compensating optical fiber having anegative dispersion slope and the connection optical fiber connectedthereto, even in the case where the mode field diameter before thefusion splicing is relatively small, since the near field pattern of thedispersion compensating optical fiber largely changes after the fusionsplicing, the joint loss increases.

[0086] The present inventors therefore have tried to reduce the jointloss between the dispersion compensating optical fiber having a negativedispersion slope and the connection optical fiber, by matching the nearfield patterns after fusion splicing, to each other.

[0087] As a result, it has been found that even in the case where thedispersion compensating optical fiber and the connection optical fiberhaving different near field patterns from each other are to beconnected, the joint loss can be reduced by matching the near fieldpatterns after fusion splicing, with each other, as described below.

[0088] For example, connection at a low loss can be realized by using aconnection optical fiber in which the theoretical joint loss obtainedfrom the overlap integral of the near field pattern of the dispersioncompensating optical fiber after fusion splicing and the near fieldpattern of the connection optical fiber after fusion splicing, ispresumed to be 0.3 dB or less, and preferably, 0.1 dB or less. If thetheoretical joint loss exceeds 0.3 dB, there is a problem in that thejoint loss increases. The theoretical joint loss is preferable as smallas possible, but from the standpoint of joint loss with the single modeoptical fiber described below, which is compensated by the dispersioncompensating optical fiber, the theoretical joint loss is substantially0.05 dB or higher.

[0089] The theoretical joint loss can be estimated from the refractiveindex distribution shape, the diffusion rate of the dopant, and the sizeof the residual stress. Actually, it is preferable to confirm thetheoretical joint loss by performing a preliminary experiment.

[0090] The theoretical joint loss can be obtained in the followingmanner.

[0091] At first, one end of the dispersion compensating optical fiber isheated in the unconnected state under the same conditions as at the timeof fusion splicing. The connection optical fiber connected to thisdispersion compensating optical fiber is similarly heated in theunconnected state.

[0092] With respect to the dispersion compensating optical fiber and theconnection optical fiber, the power distribution of the light whichenters from one end and exits from the other end is measured by a farfield pattern measuring device, using a method specified in the abovedescribed ITUT Standard, and the measurement result is subjected to aninverse Fourier transform to thereby obtain a near field pattern.

[0093] By multiplying the thus obtained near field pattern by ½, theelectric field intensity distribution can be obtained.

[0094] If the electric field intensity is substituted in expression 1described below, the theoretical joint loss obtained from the overlapintegral of these near field patterns can be calculated.

[0095] Generally, if the near field patterns are the same, thetheoretical joint loss is zero, and as the near field pattern becomessimilar, the theoretical joint loss becomes smaller. $\begin{matrix}{{- 10}\quad \log_{10}\frac{\left\{ {\int{{E_{1}(r)}{E_{2}(r)}r{r}}} \right\}^{2}}{\int{{E_{1}^{2}(r)}r{r}{\int{{E_{2}^{2}(r)}r{r}}}}}({dB})} & (1)\end{matrix}$

[0096] In expression 1, E₁(r) and E₂(r) respectively denote the electricfield intensity of the dispersion compensating optical fiber and theconnection optical fiber.

[0097] As for the near field pattern of the connection optical fiber, aconnection optical fiber in which the near field pattern does not changelargely due to heating, compared to the near field pattern of thedispersion compensating optical fiber is preferably selected.

[0098] In such a connection optical fiber, the conditions for thetheoretical joint loss can be easily satisfied, by selecting aconnection optical fiber having a near field pattern before fusionsplicing which is close to the near field pattern after fusion splicingof the dispersion compensating optical fiber, or a connection opticalfiber having a near field pattern slightly smaller than the near fieldpattern after fusion splicing of the dispersion compensating opticalfiber, taking into consideration that the dopant such as germanium addedto the core diffuses due to heating to thereby expand the near fieldpattern slightly.

[0099] In order to easily satisfy such conditions for the theoreticaljoint loss, the dispersion compensating optical fiber and the connectionoptical fiber preferably satisfy the following conditions.

[0100] That is to say, the effective core area of the connection opticalfiber in the unconnected state is preferably larger than that of thedispersion compensating optical fiber in the unconnected state.

[0101] The difference thereof is, for example, from 1 to 35 μm², andpreferably, from 2 to 25 μm². If the difference is less than 1 μm²,sufficient effect cannot be obtained, and if the difference exceeds 35μm², there is the possibility that the joint loss increases.

[0102] Also, the expansion rate of the effective core area of theconnection optical fiber due to heating is preferably smaller than thatof the effective core area of the dispersion compensating optical fiberdue to heating. If the connection optical fiber has a larger expansionrate than the dispersion compensating optical fiber, bonding to thecladding mode is likely to occur before the near field patterns arematched, and hence the joint loss tends to increase.

[0103] The expansion rate of the effective core area due to heating canbe measured in the following manner. That is, the size of the effectivecore area is obtained by the far field pattern measuring device asdescribed above, using the heating time as a parameter, under apredetermined heating temperature condition.

[0104] This expansion rate is preferably such that when values of theexpansion rate under certain heating conditions are compared, the ratioof the expansion rate of the dispersion compensating optical fiber tothe expansion rate of the connection optical fiber is from 1.1 to 8.0,and more preferably, from 1.2 to 7.0. If it is below 1.1, sufficienteffect cannot be obtained, and if it exceeds 8.0, control of the heatingtime at the time of connection may be difficult.

[0105] Since the expansion rate largely depends on the heatingconditions (particularly, on the heating temperature), it is necessaryto compare values under the same heating conditions.

[0106] From the standpoint of reducing the joint loss, it is preferableto use a connection optical fiber in which even if it is heated for along period of time at the time of fusion splicing, and the dopant suchas germanium added to the core diffuses, the propagation modepropagating the core is hardly bonded with the cladding mode. Therefore,it is preferable to select one having a large difference in thepropagation constant (Δβ) between the propagation mode and the claddingmode. Δβ is set to be, for example, 8000 (rad/m) or larger, andpreferably, 9000 (rad/m) or larger.

[0107] The connection optical fiber includes one comprising for examplea center core, a side core provided on the circumference thereof, and acladding provided on the circumference of the side core and having aso-called dual type refractive index distribution shape in which therefractive index decreases in order from the center core, the side coreand the cladding. In this dual type refractive index distribution shape,for example, the center core and the side core are formed of agermanium-added silica glass or the like. The cladding will be describedlater.

[0108] Since the connection optical fiber is appropriately selectedcorresponding to the dispersion compensating optical fiber, conditionssuch as the structural parameters are not particularly limited. However,the relative index difference of the center core based on the claddingis preferably 1.0% or less, and substantially at least 0.5%, from thestandpoint of suppressing changes in the near field pattern due toheating.

[0109] Also the relative index difference of the side core based on thecladding is preferably from 0.07 to 0.2%, from the standpoint of havinga suitable cutoff wavelength, having a suitable near field pattern andreducing bend loss.

[0110] Moreover, the ratio of the radius of the side core to the radiusof the center core is preferably from 2.5 to 4.0, from the standpoint ofhaving a suitable cutoff wavelength, having a suitable near fieldpattern and reducing bend loss.

[0111] Furthermore, there can be exemplified a connection optical fiberhaving a refractive index distribution shape similar to that shown inFIG. 3, and comprising a center core, a side core provided on thecircumference of the center core, a ring core provided on thecircumference of the side core, and a cladding provided on thecircumference of the ring core, wherein the refractive index of thecenter core and the ring core is respectively higher than that of thecladding, and the refractive index of the side core is higher than thatof the cladding and lower than that of the center core and the ringcore.

[0112] In this refractive index distribution shape, for example, thecenter core and the ring core are formed of a germanium-added silicaglass, and the side core comprises a pure silica glass, agermanium-added silica glass, or a fluorine-added silica glass. Thecladding will be described later.

[0113] Since the connection optical fiber is appropriately selectedcorresponding to the dispersion compensating optical fiber, conditionssuch as the structural parameters are not particularly limited. However,the relative index difference of the center core based on the claddingis preferably 1.2% or less, and substantially 0.8% or more, from thestandpoint of suppressing changes in the near field pattern due toheating.

[0114] Also the relative index difference of the side core based on thecladding is preferably from +0.05 to +0.10%, from the standpoint ofhaving a suitable cutoff wavelength, having a suitable near fieldpattern and reducing bend loss.

[0115] The relative index difference of the ring core based on thecladding is preferably from 0.2 to 0.4%, from the standpoint of having asuitable cutoff wavelength, having a suitable near field pattern andreducing bend loss.

[0116] The ratio of the radius of the side core to the radius of thecenter core is preferably from 3.2 to 3.8, from the standpoint of havinga suitable cutoff wavelength, having a suitable near field pattern andreducing bend loss.

[0117] Also, the ratio of the radius of the ring core to the radius ofthe center core is preferably from 4.0 to 5.0, from the standpoint ofhaving a suitable cutoff wavelength, having a suitable near fieldpattern and reducing bend loss.

[0118] The conditions for the fusion splicing between the connectionoptical fiber and the dispersion compensating optical fiber are notparticularly limited, but for example, a temperature of from 1800 to2300° C. and a period of from 0.8 to 3 seconds are preferable. It isalso preferable to perform fusion splicing, while shining light thereonand monitoring the optical characteristic. Values of the expansion rateare compared under the same heating conditions, as described above.

[0119] One end of the connection optical fiber is connected to thedispersion compensating optical fiber, but the other end thereof ispreferably fusion spliced with a single mode optical fiber having apositive chromatic dispersion and dispersion slope compensated by thisdispersion compensating optical fiber. The used length of the connectionoptical fiber at this time is, for example, 50 cm or more, andpreferably not larger than 20 m. If the length is less than 50 cm, thelength is not sufficient, and hence the operability of the fusion splicemay drop, or light corresponding to the loss occurring at the joint maybe combined with the mode propagating the core to cause noise. If thelength exceeds 20 m, a problem may occur from the standpoint of thetransmission characteristics.

[0120] For this single mode optical fiber, there is used one havingpositive chromatic dispersion and dispersion slope, and having aneffective core area of preferably at least 55 μm², and more preferablyat least 80 μm², in the used wavelength. Normal single mode opticalfibers such as one for use at 1.3 μm, and a dispersion shifted opticalfiber, which have an effective core area of from 55 to 90 μm², may beused, but one having an effective core area of from 100 to 150 μm², andpreferably from 120 to 140 μm² is particularly preferable. A single modeoptical fiber having an effective core area in this range hardly causesany nonlinear effects, and contributes to an improvement of thetransmission characteristics.

[0121] A single mode optical fiber having a large effective core areaincludes an optical fiber having a so-called W-type refractive indexdistribution shape, which comprises, for example, a center core, a sidecore provided on the circumference of the center core and having arefractive index lower than that of the center core, and a claddingprovided on the circumference of the side core and having a refractiveindex higher than that of the side core.

[0122] In order to expand the effective core area, it is preferable thatthe relative index difference of the center core based on the claddingis from 0.2 to 0.25%, the relative index difference of the side corebased on the cladding is from −0.02 to −0.07%, and the ratio of thediameter of the side core to the diameter of the center core is from 3.5to 4.5.

[0123] Which one of the single mode optical fiber having an effectivecore area of from 55 to 90 μm², or the single mode optical fiber havingan effective core area of from 100 to 150 μm² is to be used, isappropriately judged, depending on the application thereof and therequired characteristics.

[0124] In the dispersion compensating optical fiber, the connectionoptical fiber and the single mode optical fiber, when the effective corearea of the dispersion compensating optical fiber in the unconnectedstate is designated as A, the effective core area of the connectionoptical fiber in the unconnected state is designated as B, and theeffective core area of the single mode optical fiber in the unconnectedstate is designated as C, the relation thereof is preferably A<B<C.

[0125] Also, when the expansion rate of the effective core area due toheating of the dispersion compensating optical fiber is designated as D,the expansion rate of the effective core area due to heating of theconnection optical fiber is designated as E, and the expansion rate ofthe effective core area due to heating of the single mode optical fiberis designated as F, the relation thereof is preferably F<E<D. Themeasurement method of the expansion rate is as described above. Valuesof the expansion rate should be compared under the same heatingconditions, as described above.

[0126] As described above, in order to connect the dispersioncompensating optical fiber and the connection optical fiber at a lowloss, it is necessary that A<B and E<D. Generally, the relation of A<Cis established herein. Therefore, in order to connect the dispersioncompensating optical fiber, the connection optical fiber and the singlemode optical fiber all at a low loss, it is necessary that B<C and F<E.

[0127] The difference between B and C is designated herein as from 15 to130 μm², and preferably from 20 to 120 μm². If the difference is lessthan 15 μm², control of the heating conditions at the time of connectionbecomes difficult, and if it exceeds 130 μm², the joint loss between theconnection optical fiber and the single mode optical fiber may not bereduced sufficiently. Moreover, the ratio of E to F (E/F) is designatedas from 2 to 15, and preferably from 2.5 to 10. If the ratio is lessthan 2, bonding with the cladding mode occurs in the connection opticalfiber, before the joint loss between the connection optical fiber andthe single mode optical fiber is reduced. Hence, connection at a lowloss may not be realized. If the ratio exceeds 15, control of theheating conditions at the time of connection may be difficult.

[0128] There is a tendency for the connection optical fiber to have aneffective core area similar to that of the dispersion compensatingoptical fiber, in order to satisfy the above described near fieldpattern condition, and hence it has a large difference in the effectivecore area from that of the single mode optical fiber of the nonlineareffect suppression type (effective core area expansion type), asdescribed above. The effective core area of the dispersion compensatingoptical fiber having a negative dispersion slope is, for example,approximately from 16 to 35 μm².

[0129] Therefore, at the time of fusion splicing with the single modeoptical fiber, it is desirable that the connection optical fiber beheated for a relatively long period of time to diffuse the dopant suchas germanium added to the core as much as possible, to thereby expandthe effective core area, in order to reduce the joint loss between theconnection optical fiber and the single mode optical fiber.

[0130] At this time, in the case where the outer layer adjacent to thecore (on the circumference of the core) comprises a fluorine-addedsilica glass, diffusion of the dopant such as germanium can be promoted,thereby enabling rapid expansion of the effective core area.

[0131] The added amount of fluorine is preferably at least 0.6% byweight, and more preferably, from 0.9 to 1.5% by weight. If the amountis less than 0.6% by weight, the diffusion promotion effect cannot beobtained. On the other hand, if it exceeds 1.5% by weight, the nearfield pattern is likely to change at the time of fusion splicing withthe dispersion compensating optical fiber, and hence a problem mayoccur.

[0132] However, if the dopant is added, the melting point of the silicaglass decreases. Therefore, there is a problem in that the outer shapeof the connection optical fiber may be deformed due to heating for along period of time.

[0133] Hence, it is preferable that the cladding is formed by at leasttwo layers, with the layer adjacent to the core being formed of afluorine-added silica glass, and the outermost layer being formed of apure silica glass, so as to promote diffusion of the dopant and preventdeformation of the outer shape.

[0134] The outer diameter of each layer constituting the cladding can beappropriately changed by means of the effective core areas of theconnection optical fiber and the single mode optical fiber. However, itis generally preferable that the outer diameter of the first layer ofthe cladding comprising a fluorine-added silica glass, which is adjacentto the core, is generally from 45 to 70 μm, and the outer diameter ofthe outermost layer of the cladding is about 125 μm.

[0135] The heating conditions for fusion splicing and the expansion ofthe core are not particularly limited, but preferably, for example, atemperature of from 1800 to 2300° C. and a period of from 10 to 30seconds is preferable.

[0136] In the dispersion compensating optical fiber, it is preferablefrom the standpoint of improving the transmission characteristics thatthe effective core area is as large as possible.

[0137] In the dispersion compensating optical fiber having a W-typerefractive index distribution shape as shown in FIG. 2, the effectivecore area is preferably at least 16 μm² (substantially not larger than20 μm²).

[0138] In the dispersion compensating optical fiber having a W-typerefractive index distribution shape with a segment as shown in FIG. 3there is a tendency for the effective core area to be expanded more thanthe dispersion compensating optical fiber having the W-type refractiveindex distribution shape as described above. The effective core area ofthis dispersion compensating optical fiber is preferably at least 18 μm²(substantially not larger than 35 μm²).

[0139] Moreover, in the dispersion compensating optical fiber used inthe present invention, the value of the dispersion slope changesdepending on other characteristics such as the chromatic dispersion andthe effective core area. Therefore, the value of the dispersion slope isappropriately selected corresponding to the chromatic dispersion and thedispersion slope of the single mode optical fiber to be compensated bythis dispersion compensating optical fiber.

[0140] Specifically, in the present invention, for example a dispersioncompensating optical fiber having characteristics described below ispreferably used.

[0141] The dispersion compensating optical fiber of a first embodimenthas a W-type refractive index distribution shape with a segment as shownin FIG. 3, wherein the chromatic dispersion value is from −60 to −45ps/nm/km, the dispersion slope is from −0.180 to −0.135 ps/nm²/km, theeffective core area is from 20 to 26 μm², and the transmission loss isnot larger than 0.35 dB/km (substantially not smaller than 0.25 dB/km).

[0142] This dispersion compensating optical fiber has a large chromaticdispersion value, and as a result, the transmission loss is also small.It has also a characteristic that the dispersion slope and the effectivecore area are large.

[0143] Preferably, r₁₂/r₁₁ is from 2.5 to 5.0, r₁₃/r₁₁ is from 4.0 to5.5, Δ₁₁ is from 0.8 to 1.5%, Δ₁₂ is from −0.3 to −0.45%, and Δ₁₃ isfrom 0.4 to 1.0%. It is preferable to combine and select valuessatisfying the above described preferable characteristics from thesenumerical ranges.

[0144] The dispersion compensating optical fiber in a second embodimenthas a W-type refractive index distribution shape with a segment as shownin FIG. 3, wherein the chromatic dispersion value is from −45 to −35ps/nm/km, the dispersion slope is from −0.150 to −0.100 ps/nm²/km, theeffective core area is from 26 to 35 μm², and the transmission loss isnot larger than 0.25 dB/km (substantially not smaller than 0.20 dB/km).

[0145] This dispersion compensating optical fiber has a smallerchromatic dispersion value than the chromatic dispersion value of thedispersion compensating optical fiber of a first embodiment, and as aresult, the transmission loss is further small. Furthermore, it has alsoa characteristic that the dispersion slope and the effective core areaare further large.

[0146] In wavelength multiplex transmission, since it is important thatsmall transmission loss and large effective core area are achieved, theabove second embodiment is preferably adopted.

[0147] Preferably, r₁₂/r₁₁ is from 2.5 to 5.0, r₁₃/r₁₁ is from 4.0 to5.5, Δ₁₁ is from 0.8 to 1.5%, Δ₁₂ is from −0.3 to −0.45%, and Δ₁₃ isfrom 0.4 to 1.0%. It is preferable to combine and select valuessatisfying the above described preferable characteristics from thesenumerical ranges.

[0148] The dispersion compensating optical fiber in a third embodimenthas a W-type refractive index distribution shape with a segment as shownin FIG. 3, wherein the chromatic dispersion value is from −100 to −80ps/nm/km, the dispersion slope is from −0.300 to −0.230 ps/nm²/km, theeffective core area is from 18 to 24 μm², and the transmission loss isnot larger than 0.40 dB/km (substantially not smaller than 0.31 dB/km).

[0149] This dispersion compensating optical fiber has a large chromaticdispersion value, and as a result, the transmission loss is relativelylarge. Moreover, in this dispersion compensating optical fiber there isa tendency for the dispersion slope and the effective core area to beslightly smaller, compared to the dispersion compensating optical fiberof the first embodiment.

[0150] Preferably, r₁₂/r₁₁ is from 2.5 to 4.0, r₁₃/r₁₁ is from 2.7 to8.0, Δ₁₁ is from 1.2 to 1.7%, Δ₁₂ is from −0.25 to −0.45%, and Δ₁₃ isfrom 0.2 to 1.1%. It is preferable to combine and select valuessatisfying the above described preferable characteristics from thesenumerical ranges.

[0151]FIG. 4 is a graph showing an example of a near field pattern ofeach optical fiber used for the connection structure of the presentinvention.

[0152] No. 1 is a normal single mode optical fiber such as one for useat 1.3 μm (the effective core area being 80 μm²).

[0153] No. 2 is a single mode optical fiber having the effective corearea of 135 μm².

[0154] No. 3 is a dispersion compensating optical fiber heated under thesame conditions as that of the fusion splice.

[0155] No. 4 is a near field pattern of a connection optical fiberheated under the same conditions as that of the fusion splice.

[0156] The near field patterns of No. 3 and No. 4 coincide well witheach other, and the theoretical joint loss in this example is 0.08 dB.

[0157] In a transmission line having the connection structure of thepresent invention, the dispersion compensating optical fiber and thesingle mode optical fiber can be connected at a low loss via theconnection optical fiber, thereby enabling improvement of thetransmission characteristics.

[0158] The connection structure of the present invention can be alsoapplied to a dispersion compensator.

[0159] That is to say, the dispersion compensator is for providing thedispersion compensating optical fiber in a module. For example, thismodule accommodates the dispersion compensating optical fiber wound in acylindrical body inside a hexahedron housing or the like.

[0160] In the present invention, for example a dispersion compensatorcan be constructed in such a manner that connection optical fibers aslead fibers are respectively fusion-spliced to the opposite ends of thedispersion compensating optical fiber in a housing, and these leadfibers are respectively pulled out from two holes provided in thehousing. By connecting a single mode optical fiber used for atransmission line to these pulled out lead fibers, a low-losstransmission line can be constructed.

[0161] The cylindrical body and the housing used for the dispersioncompensator are formed of, for example, a metal or ceramics, and thesize thereof can be appropriately changed depending on the length of thedispersion compensating optical fiber.

[0162] In the transmission line and the dispersion compensator, thedispersion compensating optical fiber, the single mode optical fiber andthe connection optical fiber (lead fiber) are used in the form of anoptical fiber strand in which a covering layer comprising an ultraviolethardening-type resin is provided on the circumference of the outermostlayer (cladding) consisting of a silica glass, or in the form of anoptical fiber core in which a covering layer comprising a nylon or thelike is further provided on the circumference of the optical fiberstrand.

EXAMPLES

[0163] The present invention will now be described in detail by way ofexamples.

Example 1

[0164] A dispersion compensating optical fiber having a negativedispersion slope and a single mode optical fiber compensated by thisdispersion compensating optical fiber were fusion-spliced to theopposite ends of a connection optical fiber, and the near field patternand the joint loss were measured. The used wavelength was 1.55 μm, andthe outer diameters of the used optical fibers (outer diameter ofcladding) were respectively about 125 μm. Since the expansion rate ofthe effective core area due to heating largely depends on the heatingtemperature, the values in this example are shown as reference values.The heating conditions were substantially the same.

[0165] The dispersion compensating optical fiber had a W-type refractiveindex distribution shape with a segment as shown in FIG. 3, and thestructural parameters and characteristics thereof were as follows.

[0166] The center core and the ring core were made of a germanium-addedsilica glass, the side core was made of a fluorine-added silica glass,and the cladding was made of a pure silica glass.

[0167] r₁₁: 1.8 μm

[0168] r₁₂: 5.8 μm

[0169] r₁₃: 7.1 μm

[0170] Δ₁₁: 1.65%

[0171] Δ₁₂: −0.35%

[0172] Δ₁₃: 0.5%

[0173] Effective core area: 22 μm²

[0174] Mode field diameter: 5.3 μm

[0175] Chromatic dispersion: −93 ps/nm/km

[0176] Dispersion slope: −0.28 ps/nm²/km

[0177] Transmission loss: 0.33 dB/km

[0178] Cutoff wavelength: 1.7 μm

[0179] Expansion rate of the effective core area due to heating: 7.0μm²/sec

[0180] The connection optical fiber had a W-type refractive indexdistribution shape with a segment, and one having a near field patterndifferent from that of the dispersion compensating optical fiber beforefusion splicing was used (theoretical joint loss before fusion splicingwas 0.7 dB). The cladding had a one-layer structure, and was formed of apure silica glass. The structural parameters and characteristics thereofwere as follows.

[0181] The center core was made of a germanium-added silica glass, theside core was made of a silica glass in which fluorine and germaniumwere co-added, and the ring core was made of a germanium-added silicaglass.

[0182] Relative index difference of the center core based on thecladding: 1.3%

[0183] Relative index difference of the side core based on the cladding:0.01%

[0184] Relative index difference of the ring core based on the cladding:0.35%

[0185] Radius of the center core: 1.2 μm

[0186] Radius of the side core: 8.3 μm

[0187] Radius of the ring core: 9.4 μm

[0188] Mode field diameter: 6.2 μm

[0189] Effective core area: 24.5 μm²

[0190] Expansion rate of the effective core area due to heating: 3.0μm²/sec

[0191] The single mode optical fiber had a W-type refractive indexdistribution shape with a segment as shown in FIG. 3, and the structuralparameters and characteristics thereof were as follows.

[0192] The center core was made of a germanium-added silica glass, theside core was made of a fluorine-added silica glass, and the claddingwas made of a pure silica glass.

[0193] Relative index difference of the center core based on thecladding: 0.25%

[0194] Relative index difference of the side core based on the cladding:−0.05%

[0195] Radius of the center core: 6.8 μm

[0196] Radius of the side core: 27 μm

[0197] Mode field diameter: 12.7 μm

[0198] Effective core area: 135 μm²

[0199] Chromatic dispersion: 20 ps/nm/km

[0200] Dispersion slope: 0.06 ps/nm²/km

[0201] Transmission loss: 0.19 dB/km

[0202] Cutoff wavelength: 1.6 μm

[0203] Expansion rate of the effective core area due to heating: 2.0μm²/sec

[0204] A fusion splicing machine was used for connection of theconnection optical fiber with the dispersion compensating optical fiber,to heat the connection optical fiber at about 2200° C. for 2 seconds.The mode field diameter of the dispersion compensating optical fiberafter fusion splicing was 5.9 μm. The mode field diameter of theconnection optical fiber after fusion splicing hardly changed.

[0205] A fusion splicing machine was used for connection of theconnection optical fiber with the single mode optical fiber, to heat theconnection optical fiber at about 2200° C. for 2 seconds for effectingfusion splicing, and thereafter, the end of the connection optical fiberwas further heated for 30 seconds to thereby diffuse the germanium.

[0206] The theoretical joint loss obtained from the overlap integral ofthe near field pattern of the dispersion compensating optical fiber andthe near field pattern of the connection optical fiber was 0.11 dB. Theactual measurement values of the joint loss are shown in Table 1.

Example 1-A

[0207] As another single mode fiber, a single mode optical fiber had astep type refractive index distribution shape, and the structuralparameters and characteristics thereof were as follows.

[0208] Relative index difference of the core based on the cladding:0.31%

[0209] Radius of the core: 4.5 μm

[0210] Mode field diameter: 10.1 μm

[0211] Effective core area: 83 μm²

[0212] Chromatic dispersion: 16.5 ps/nm/km

[0213] Dispersion slope: 0.057 ps/nm²/km

[0214] Transmission loss: 0.196 dB/km

[0215] Cutoff wavelength: 1.2 μm

[0216] Expansion rate of the effective core area due to heating: 1.1μm²/sec

[0217] A fusion splicing machine was used for connection of theconnection optical fiber with the single mode optical fiber, to heat theconnection optical fiber at about 2200° C. for 2 seconds for effectingfusion splicing, and thereafter, the end of the connection optical fiberwas further heated for 20 seconds to thereby diffuse the germanium. Theactual measurement values of the joint loss are shown in Table 1.

Example 1-B

[0218] As another single mode fiber, a single mode optical fiber had astep type refractive index distribution shape, and the structuralparameters and characteristics thereof were as follows.

[0219] Relative index difference of the core based on the cladding:0.33%

[0220] Radius of the core: 4.78 μm

[0221] Mode field diameter: 10.8 μm

[0222] Effective core area: 90 μm²

[0223] Chromatic dispersion: 18.2 ps/nm/km

[0224] Dispersion slope: 0.06 ps/nm²/km

[0225] Transmission loss: 0.196 dB/km

[0226] Cutoff wavelength: 1.34 μm

[0227] Expansion rate of the effective core area due to heating: 1.1μm²/sec

[0228] A fusion splicing machine was used for connection of theconnection optical fiber with the single mode optical fiber, to heat theconnection optical fiber at about 2200° C. for 2 seconds for effectingfusion splicing, and thereafter, the end of the connection optical fiberwas further heated for 25 seconds to thereby diffuse the germanium. Theactual measurement values of the joint loss are shown in Table 1.

Example 1-C

[0229] As another single mode fiber, a single mode optical fiber had aW-type refractive index distribution shape with a segment, and thestructural parameters and characteristics thereof were as follows.

[0230] Relative index difference of the center core based on thecladding: 0.5%

[0231] Relative index difference of the side core based on the cladding:−0.11%

[0232] Relative index difference of the ring core based on the cladding:0.18%

[0233] Radius of the center core: 3.5 μm

[0234] Radius of the side core: 5.9 μm

[0235] Radius of the ring core: 7.9 μm

[0236] Mode field diameter: 8.5 μm

[0237] Effective core area: 55 μm²

[0238] Chromatic dispersion: 3 ps/nm/km

[0239] Dispersion slope: 0.05 ps/nm²/km

[0240] Transmission loss: 0.210 dB/km

[0241] Cutoff wavelength: 1.25 μm

[0242] Expansion rate of the effective core area due to heating: 1.1μm²/sec

[0243] A fusion splicing machine was used for connection of theconnection optical fiber with the single mode optical fiber, to heat theconnection optical fiber at about 2200° C. for 2 seconds for effectingfusion splicing, and thereafter, the end of the connection optical fiberwas further heated for 10 seconds to thereby diffuse the germanium. Theactual measurement values of the joint loss are shown in Table 1.

Example 2

[0244] A dispersion compensating optical fiber having a negativedispersion slope and a single mode optical fiber compensated by thisdispersion compensating optical fiber were fusion-spliced to theopposite ends of a connection optical fiber, and the near field patternand the joint loss were measured. The used wavelength was 1.55 μm, andthe outer diameters of the used optical fibers (outer diameter ofcladding) were respectively about 125 μm. Since the expansion rate ofthe effective core area due to heating largely depends on the heatingtemperature, the values in this example are shown as reference values.The heating conditions were substantially the same.

[0245] The dispersion compensating optical fiber had a W-type refractiveindex distribution shape with a segment as shown in FIG. 3, and thestructural parameters and characteristics thereof were as follows.

[0246] The center core and the ring core were made of a germanium-addedsilica glass, the side core was made of a fluorine-added silica glass,and the cladding was made of a fluorine-added silica glass.

[0247] r₁₁: 2.0 μm

[0248] r₁₂: 5.7 μm

[0249] r₁₃: 6.9 μm

[0250] Δ₁₁: 0.8%

[0251] Δ₁₂: −0.37%

[0252] Δ₁₃: 0.4%

[0253] Effective core area: 29 μm²

[0254] Mode field diameter: 6.1 μm

[0255] Chromatic dispersion: −40 ps/nm/km

[0256] Dispersion slope: −0.21 ps/nm²/km

[0257] Transmission loss: 0.228 dB/km

[0258] Cutoff wavelength: 1.5 μm

[0259] Expansion rate of the effective core area due to heating: 8.3μm²/sec

[0260] The cladding in the connection optical fiber had a two-layerstructure, with the first layer adjacent to the core being formed of asilica glass added with fluorine in an amount of 2% by weight and theradius thereof was 25 μm. The outermost layer (the second layer) wasformed of a pure silica glass.

[0261] Relative index difference of the center core based on the firstlayer of the cladding: 1.9%

[0262] Relative index difference of the side core based on the firstlayer of the cladding: 0.05%

[0263] Relative index difference of the ring core based on the firstlayer of the cladding: 0.38%

[0264] Radius of the center core: 1.9 μm

[0265] Radius of the side core: 6.9 μm

[0266] Radius of the ring core: 8.5 μm

[0267] Mode field diameter: 6.3 μm

[0268] Effective core area: 35 μm²

[0269] Expansion rate of the effective core area due to heating: 4.3μm²/sec

[0270] The single mode optical fiber had a W-type refractive indexdistribution shape with a segment as shown in FIG. 3, and the structuralparameters and characteristics thereof were as follows.

[0271] The center core was made of a germanium-added silica glass, theside core was made of a fluorine-added silica glass, and the claddingwas made of a pure silica glass.

[0272] Relative index difference of the center core based on thecladding: 0.25%

[0273] Relative index difference of the side core based on the cladding:−0.05%

[0274] Radius of the center core: 6.8 μm

[0275] Radius of the side core: 27 μm

[0276] Mode field diameter: 12.7 μm

[0277] Effective core area: 135 μm²

[0278] Chromatic dispersion: 20 ps/nm/km

[0279] Dispersion slope: 0.06 ps/nm²/km

[0280] Transmission loss: 0.19 dB/km

[0281] Cutoff wavelength: 1.6 μm

[0282] Expansion rate of the effective core area due to heating: 2.0μm²/sec

[0283] A fusion splicing machine was used for connection of theconnection optical fiber with the dispersion compensating optical fiber,to heat the connection optical fiber at about 2200° C. for 2 seconds.The mode field diameter of the dispersion compensating optical fiberafter fusion splicing was 6.4 μm. The mode field diameter of theconnection optical fiber after fusion splicing hardly changed.

[0284] A fusion splicing machine was used for connection of theconnection optical fiber with the single mode optical fiber, to heat theconnection optical fiber at about 2200° C. for 2 seconds for effectingfusion splicing, and thereafter, the end of the connection optical fiberwas further heated for 30 seconds to thereby diffuse the germanium.

[0285] The theoretical joint loss obtained from the overlap integral ofthe near field pattern of the dispersion compensating optical fiber andthe near field pattern of the connection optical fiber was 0.11 dB. Theactual measurement values of the joint loss are shown in Table 1.

Example 3

[0286] The connection structure was constructed in the same manner as inExample 1, except that the construction of the cladding of theconnection optical fiber was changed.

[0287] That is to say, the cladding in the connection optical fiber hada two-layer structure, with the first layer adjacent to the core beingformed of a silica glass added with fluorine in an amount of 1.2% byweight and the radius thereof was 25 μm. The outermost layer (the secondlayer) was formed of a pure silica glass.

[0288] The structural parameters and characteristics of this connectionoptical fiber were as follows.

[0289] Relative index difference of the center core based on the firstlayer of the cladding: 1.0%

[0290] Relative index difference of the side core based on the firstlayer of the cladding: 0.05%

[0291] Relative index difference of the ring core based on the firstlayer of the cladding: 0.38%

[0292] Radius of the center core: 1.8 μm

[0293] Radius of the side core: 6.7 μm

[0294] Radius of the ring core: 8.2 μm

[0295] Mode field diameter: 5.7 μm

[0296] Effective core area: 24.7 μm²

[0297] Expansion rate of the effective core area due to heating: 5.6μm²/sec

[0298] The mode field diameter of the dispersion compensating opticalfiber after fusion splicing was 5.9 μm. The mode field diameter of theconnection optical fiber after fusion splicing also changed, and was 5.9μm.

[0299] The theoretical joint loss obtained from the overlap integral ofthe near field pattern of the dispersion compensating optical fiber andthe near field pattern of the connection optical fiber was 0.07 dB. Theactual measurement values of the joint loss are shown in Table 1.

Comparative Example 1

[0300] A dispersion compensating optical fiber having a negativedispersion slope and a single mode optical fiber compensated by thisdispersion compensating optical fiber were fusion-spliced to theopposite ends of a connection optical fiber, in the same manner as inExample 1, and the near field pattern and the joint loss were measured.The used wavelength was 1.55 μm, and the outer diameters of the usedoptical fibers were respectively about 125 μm.

[0301] The dispersion compensating optical fiber had a W-type refractiveindex distribution shape with a segment as shown in FIG. 3, and thestructural parameters and characteristics thereof were as follows.

[0302] The center core and the ring core were made of a germanium-addedsilica glass, the side core was made of a fluorine-added silica glass,and the cladding was made of a pure silica glass.

[0303] r₁₁: 2.0 μm

[0304] r₁₂: 5.8 μm

[0305] r₁₃: 6.8 μm

[0306] Δ₁₁: 1.0%

[0307] Δ₁₂: −0.4%

[0308] Δ₁₃: 0.9%

[0309] Effective core area: 26 μm²

[0310] Mode field diameter: 6.0 μm

[0311] Chromatic dispersion: −54 ps/nm/km

[0312] Dispersion slope: −0.15 ps/nm²/km

[0313] Transmission loss: 0.3 dB/km

[0314] Cutoff wavelength: 1.6 μm

[0315] Expansion rate of the effective core area due to heating: 8.5μm²/sec

[0316] The connection optical fiber had a step type refractive indexdistribution shape, and one having a near field pattern different fromthat of the dispersion compensating optical fiber before fusion splicingwas used (theoretical joint loss before fusion splicing was 0.6 dB). Thestep type is one having a two-layer structure consisting of a core and acladding provided on the circumference thereof, wherein the refractiveindex of the core is higher than that of the cladding.

[0317] The cladding had a two-layer structure, with the first layerbeing formed of a fluorine-added silica glass in which fluorine wasadded in an amount of 0.4% by weight, and the radius thereof was 30 μm.The outermost layer (the second layer) was formed of a pure silicaglass.

[0318] The structural parameters and characteristics of this connectionoptical fiber were as follows.

[0319] The core was made of a germanium-added silica glass.

[0320] Relative index difference of the core based on the first layer ofthe cladding: 1.0%

[0321] Radius of the core: 2.4 μm

[0322] Mode field diameter: 6.2 μm

[0323] Effective core area: 29 μm²

[0324] Expansion rate of the effective core area due to heating: 7.2μm²/sec

[0325] A single mode optical fiber the same as in Example 1 was usedherein.

[0326] The theoretical joint loss obtained from the overlap integral ofthe near field pattern of the dispersion compensating optical fiber andthe near field pattern of the connection optical fiber after theconnection was 0.95 dB. The actual measurement values of the joint lossare shown in Table 1.

Comparative Example 2

[0327] The connection optical fiber had a step type refractive indexdistribution shape, and one having a near field pattern different fromthat of the dispersion compensating optical fiber before fusion splicingwas used (theoretical joint loss before fusion splicing was 0.68 dB).The cladding had a two-layer structure, with the first layer beingformed of a fluorine-added silica glass in which fluorine was added inan amount of 0.3% by weight, and the radius thereof was 30 μm. Theoutermost layer (the second layer) was formed of a pure silica glass.

[0328] The structural parameters and characteristics of this connectionoptical fiber were as follows.

[0329] The core was made of a germanium-added silica glass.

[0330] Relative index difference of the core based on the first layer ofthe cladding: 1.2%

[0331] Radius of the core: 3.0 μm

[0332] Mode field diameter: 6.0 μm

[0333] Effective core area: 27 μm²

[0334] Expansion rate of the effective core area due to heating: 7.1μm²/sec

[0335] A dispersion compensating optical fiber and a single mode opticalfiber the same as in Comparative Example 1 was used herein.

[0336] The theoretical joint loss obtained from the overlap integral ofthe near field pattern of the dispersion compensating optical fiber andthe near field pattern of the connection optical fiber after theconnection was 0.90 dB. The actual measurement values of the joint lossare shown in Table 1. TABLE 1 Joint loss with dispersion Joint loss withsingle Total joint loss compensating mode optical fiber (dB) opticalfiber (dB) (dB) Example 1 0.71 0.18 0.53 Example 1-A 0.33 0.18 0.15Example 1-B 0.36 0.18 0.18 Example 1-C 0.33 0.18 0.15 Example 2 0.370.13 0.24 Example 3 0.38 0.14 0.24 Com. Ex. 1 1.08 0.93 0.15 Com. Ex. 21.09 0.92 0.17

[0337] Examples 1, 1-A to 1-C, and 2 are compared with ComparativeExample 1.

[0338] The difference in the mode field diameter between the dispersioncompensating optical fiber and the connection optical fiber beforefusion splicing in Examples 1, 1-A to 1-C, and 2 was larger than thatfor Comparative Examples 1 and 2.

[0339] However, as the results shown in Table 1, in Examples 1, 1-A to1-C, and 2, the joint loss could be reduced more than for theComparative Examples 1 and 2. Particularly, in Examples 2 and 3 wherefluorine was added to the cladding respectively, the joint loss with thesingle mode optical fiber could be reduced considerably. Therefore, itis apparent that it is important to match the near field patterns afterfusion splicing with each other, in the connection between thedispersion compensating optical fiber having a negative dispersion slopeand the connection optical fiber.

[0340] For comparison sake, the dispersion compensating optical fiberand the single mode optical fiber used in each of the Examples andComparative Examples were directly fusion-spliced. The joint lossthereof had a value exceeding 1.5 dB.

What is claimed is:
 1. A connection structure for a dispersioncompensating optical fiber obtained by fusion-splicing a dispersioncompensating optical fiber having a negative dispersion slope with aconnection optical fiber having a different near field pattern from thatof the dispersion compensating optical fiber, wherein said connectionoptical fiber has a near field pattern such that a theoretical jointloss in a used wavelength obtained from an overlap integral of a nearfield pattern of said dispersion compensating optical fiber after fusionsplicing and a near field pattern of said connection optical fiber afterfusion splicing is 0.3 dB or less, in an unconnected state.
 2. Aconnection structure for a dispersion compensating optical fiberaccording to claim 1, wherein an effective core area of said connectionoptical fiber in the unconnected state is larger than that of saiddispersion compensating optical fiber in the unconnected state.
 3. Aconnection structure for a dispersion compensating optical fiberaccording to claim 1, wherein an expansion rate of an effective corearea due to heating of said connection optical fiber is smaller thanthat of an effective core area due to heating of said dispersioncompensating optical fiber.
 4. A connection structure for a dispersioncompensating optical fiber according to claim 1, wherein said dispersioncompensating optical fiber is connected to one end of said connectionoptical fiber, and an other end of said connection optical fiber isconnected to a single mode optical fiber having a positive dispersionslope which is compensated by said dispersion compensating opticalfiber.
 5. A connection structure for a dispersion compensating opticalfiber according to claim 4, wherein when an effective core area of saiddispersion compensating optical fiber in the unconnected state isdesignated as A, an effective core area of said connection optical fiberin the unconnected state is designated as B, and an effective core areaof said single mode optical fiber in the unconnected state is designatedas C, a relation thereof is A<B<C.
 6. A connection structure for adispersion compensating optical fiber according to claim 4, wherein whenan expansion rate of an effective core area due to heating of saiddispersion compensating optical fiber is designated as D, an expansionrate of an effective core area due to heating of said connection opticalfiber is designated as E, and an expansion rate of an effective corearea due to heating of said single mode optical fiber is designated asF, a relation thereof is F<E<D.
 7. A connection structure for adispersion compensating optical fiber according to claim 4, wherein aneffective core area in a used wavelength of said single mode opticalfiber is from 100 to 150 μm².
 8. A connection structure for a dispersioncompensating optical fiber according to claim 4, wherein an effectivecore area in a used wavelength of said single mode optical fiber is from55 to 90 μm².
 9. A connection structure for a dispersion compensatingoptical fiber according to claim 1, wherein said connection opticalfiber comprises a core and a cladding provided on a circumference ofsaid core, and fluorine is added to said cladding.
 10. A connectionstructure for a dispersion compensating optical fiber according to claim9, wherein the addition of the fluorine is at least 0.6% by weight. 11.A connection structure for a dispersion compensating optical fiberaccording to claim 9, wherein said cladding comprises at least twolayers, and an outermost layer of said cladding comprises pure silica.12. A connection structure for a dispersion compensating optical fiberaccording to claim 1, wherein said dispersion compensating optical fibercomprises a center core, a side core provided on a circumference of saidcenter core, and a cladding provided on a circumference of said sidecore, and wherein a refractive index of said center core is higher thanthat of said cladding, and a refractive index of said side core is lowerthan that of said cladding.
 13. A connection structure for a dispersioncompensating optical fiber according to claim 12, wherein an effectivecore area in a used wavelength of said dispersion compensating opticalfiber is 16 μm² or larger.
 14. A connection structure for a dispersioncompensating optical fiber according to claim 1, wherein said dispersioncompensating optical fiber comprises a center core, a side core providedon a circumference of said center core, a ring core provided on acircumference of said side core, and a cladding provided on acircumference of said ring core, and wherein a refractive index of saidcenter core and said ring core is respectively higher than that of saidcladding, and a refractive index of said side core is lower than that ofsaid cladding.
 15. A connection structure for a dispersion compensatingoptical fiber according to claim 14, wherein an effective core area in aused wavelength of said dispersion compensating optical fiber is 18 μm²or larger.
 16. A connection structure for a dispersion compensatingoptical fiber according to claim 15, wherein said dispersioncompensating optical fiber has characteristics shown in the following(a-1) to (d-1): (a-1) a chromatic dispersion value in the usedwavelength is from −60 to −45 ps/nm/km; (b-1) a dispersion slope in theused wavelength is from −0.180 to −0.135 ps/nm²/km; (c-1) the effectivecore area in the used wavelength is from 20 to 26 μm²; and (d-1) atransmission loss in the used wavelength is 0.35 dB/km or less.
 17. Aconnection structure for a dispersion compensating optical fiberaccording to claim 15, wherein said dispersion compensating opticalfiber has characteristics shown in the following (a-2) to (d-2): (a-2) achromatic dispersion value in the used wavelength is from −100 to −80ps/nm/km; (b-2) a dispersion slope in the used wavelength is from −0.300to −0.230 ps/nm²/km; (c-2) the effective core area in the usedwavelength is from 18 to 24 μm²; and (d-2) a transmission loss in theused wavelength is 0.40 dB/km or less.
 18. A connection structure for adispersion compensating optical fiber according to claim 15, whereinsaid dispersion compensating optical fiber has characteristics shown inthe following (a-3) to (d-3): (a-3) a chromatic dispersion value in theused wavelength is from −45 to −35 ps/nm/km; (b-3) a dispersion slope inthe used wavelength is from −0.150 to −0.100 ps/nm²/km; (c-3) theeffective core area in the used wavelength is from 26 to 35 μm²; and(d-3) a transmission loss in the used wavelength is 0.25 dB/km or less.19. A transmission line having a connection structure for a dispersioncompensating optical fiber as disclosed in any one of claim 1 throughclaim
 18. 20. A dispersion compensator having a connection structure fora dispersion compensating optical fiber as disclosed in any one of claim1 through claim
 18. 21. A connection method for a dispersioncompensating optical fiber for obtaining a connection structure for adispersion compensating optical fiber as disclosed in any one of claim 1through claim 18.